1. Field of the Invention
The present invention concerns a screen system intended for the screening of wood chips, comprising one or several screening faces placed one above the other as well as a bottom face placed underneath said screening faces, the screening faces being connected to members that produce the screening movement.
2. Description of Related Art
Prior-art screens intended for the screening of wood chips have been described in the Finnish Patents 51,775, 65,925 and 68,988. These screens comprise three screening planes placed one above the other, wherein the perforations become smaller when moving from the top towards the bottom, as well as a solid plane placed underneath the screening planes. The screen basket may be an integrated construction, or it may be divided into two blocks of two planes each. In both cases, however, the solid bottom plane is a part of the screen basket and moves along with the screening planes proper as they perform the screening movement.
When wood chips are being screened, the finest, dust-like fraction falls to the bottom plane. The bottom plane is more problematic than the perforated screen plate planes proper, on which the coarser material moves. Under certain circumstances, a deposit is formed on the bottom plane, which may block the whole space between the bottom plane and the screen plate placed above the bottom plane. The blocking causes interruptions in the screening. Moreover, the increase in the swinging mass caused by the deposits strains the construction of the screen and reduces the service life of the screen.
The deposit is formed mainly out of three reasons:
1. Resin deposit. The resinous materials contained in the fine fraction act as an adhesive and cause a rather slow increase in the deposit. Increase in the deposit resulting from resin deposition always occurs when conifer chips are being screened.
2. Freezing. Occurs in particular in screening plants into which the chips are taken from heaps stored outdoors including the snow in winter. The increase in the deposit is often very rapid.
3. Paste formation. Like the former phenomenon, this one also takes place in particular in winter when chips that contain snow are screened. In the presence of a suitable moisture content, the fine wood dust forms a dough-like paste, whereby the deposit increases rapidly.
This problem has always existed, and it has required removal of the frozen deposits by means of hot water or steam as well as removal of the resinous deposits mechanically and by means of solvents.
Attempts have been made to prevent these problems, e.g., by means of teflon-coating of the bottom, by means of chains installed on the bottom, by coating the bottom with polyurethane, as well as by heating the bottom. None of the methods that have been tested eliminates the problems. Heating of the bottom by means of electricity is employed in practical operation. It melts the ice when the screen runs, but it does not prevent resin deposition.